1. Field of the Invention
The present invention relates generally to dental prosthetics, and more particularly to retaining screws to secure dental components, such as an abutment and/or prosthesis to a dental implant or fixture.
2. Description of the Related Art
In restorative dental implantology, an artificial root or xe2x80x9cfixturexe2x80x9d is surgically implanted into the jawbone of a patient. The implant is typically formed of titanium or a commercially pure titanium alloy. Titanium, and its biocompatible alloys, referred to herein simply as titanium, is the material of choice for surgical implants because it offers a combination of nearly ideal characteristicsxe2x80x94high strength, light weight, bio-compatibility and essentially total resistance to corrosion in contact with tissues and bones.
After implanting and following an initial healing period of a few months, new bone growth is observed to occur around the titanium fixture supporting it securely in place. This process is known as osseointegration. In a secondary procedure, an abutment of a specific desired size and shape is then placed over the fixture and secured thereto by a bolt threaded into a cavity in the fixture. Typically, a titanium bolt is used, called the abutment retaining screw. The prosthesis is the third component of the system and may be fabricated of cast gold alloy and porcelain. However, since machined parts have greater accuracy than cast parts, the prosthesis is commonly cast to a machined component which is fastened to a threaded cavity in the abutment by a retaining screw.
Alternative approaches include attaching the prosthesis directly to the fixture without any intervening abutment. This method uses only one titanium retaining screw. A further variation includes cementing the restoration to the implant and/or abutment in a similar manner as is done in conventional fixed bridges on natural teeth. Typically, in this case, a tapered abutment without threads, often referred to as a cementable abutment, is fastened to the fixture with a titanium abutment retaining screw. This method also utilizes only one screw in the system.
Thus, implant dentistry relies upon one or more screws to fasten together component stacks. These components typically include fixtures, abutments and artificial teeth though they can include other components, for example, impression copings. Retaining screws have been used in dental prostheses for more than two decades and the problems inherent with screw securement have been studied in depth.
Typically, a special torque wrench is used to torque the abutment screw to the desired load or torque. However, the torque wrench measures total torque resistance only. As the abutment screw is turned, resistance increases indicating rising torque values on a dial gauge. But only part of this resistance comes about by the tension forces which are created as the abutment screw stretches under an applied load. The resistance forces which produce the clamping force are equal in magnitude and opposite in direction to the stretching force (tensile load). The important parameter of interest, however, is the resulting clamping load, which is produced when the screw is tightened at various torque values. As, noted above, this is a small component of the overall torque applied to a titanium screw, due to the large friction forces involved.
Thus, it is desirable to provide a high preload to the abutment screw so that the dental restoration can withstand the applied loads, and hence prevent joint failure and also to prevent screw loosening. Screw loosening generally occurs due to a xe2x80x9cratchetingxe2x80x9d effect as significant loads, for example, during mastication, are repeatedly applied to the dental restoration, and causes the screw threads to turn or xe2x80x9cback offxe2x80x9d in steps. As the screw loosens, the preload decreases and this renders the implant-abutment joint further susceptible to failure.
The high loads encountered by the abutment screw can also cause the contacting surfaces of the components to open slightly on one side of the implant system by bending of the screw. This can create small gaps between the opposed surfaces of the abutment and the implant. Undesirably, oral fluids may gain access to the interior of the implant system through the gaps, thus risking infection. Movement of the implant components may also cause the screws to loosen or fail as they are repeatedly stretched and bent.
U.S. Pat. No. 5,482,463 to Wilson, et al. describes a screw joint for dental implant component stacks which has greater resistance to loosening, and therefore greater resistance to bending and/or breaking. This is accomplished by the use of spring washers and has the effect of increasing the preload and of increasing resistance to slippage of the entire joint. While the use of springs, as described by Wilson, et al. provides a partial solution to the long-standing problem of loosening of screws in dental implants, the fundamental problem of low preloading remains largely unsolved.
U.S. Pat. No. 5,711,669 to Hurson, incorporated by reference herein, teaches a system for improving the tightening efficiency of a titanium abutment screw using a malleable material, such as gold or silver, that is both biocompatible and has a low coefficient of friction. However, such coatings are expensive and, in some cases, can reduce the friction to such an extent that the screws can be easily over-torqued by the dental practitioner applying only minimal torque. Moreover, the malleability of the coating can render the threads of the titanium screw susceptible to galling.
Accordingly, it is an object and advantage of the prevent invention to overcome some or all of the above limitations by providing an amorphous xe2x80x9chard carbonxe2x80x9d coated retaining screw for a dental prosthetic implant system. Though there are a wide variety of commercially available xe2x80x9chard carbonxe2x80x9d coatings and some of the properties of hard carbon as a surface coating are known in the art, the present invention provides certain novel and unique benefits and advantages over the prior art in the field of screw retained prosthetic constructions particularly in the field of oral restorations, and more particularly in the field of dental screws for retaining abutments and/or dental restorations as related to the stacking and assembly of dental components.
The hard carbon coating provides a low friction surface finish which advantageously results in improved preloading of the screw, and hence a high clamping force between the components of the dental prosthetic implant system. The coating can comprise diamond-like carbon (DLC), amorphous diamond, crystalline diamond, or a combination thereof. The dental screw can include abutment retaining screws and prosthesis retaining screws. Other advantages provided by the hard carbon coating include high mechanical surface hardness, biocompatibilty, corrosion resistance, chemical inertness and low cost.
In accordance with one embodiment of the present invention, a retaining screw for fastening a dental component to an implant is provided. The implant has osseointegrated in a jawbone. The retaining screw generally comprises a head and a shank. The screw head comprises a seating surface sized and configured to engage a seating surface of the dental component. The screw head further comprises a cavity adapted to receive a tool for tightening the screw. The shank is in mechanical communication with the head and comprises threads adapted to threadably engage a threaded socket of the implant. A coating of hard carbon is applied to the seating surface of the screw head and to at least a portion of the shank. This reduces friction during tightening of the retaining screw and improves the preloading of the screw.
In accordance with another embodiment of the present invention, a dental implant system for supporting a prosthesis is provided. The dental implant system generally comprises a retaining screw, a dental implant and an abutment. The retaining screw generally comprises a head and a shank. The screw head comprises a seating surface and a cavity adapted to receive a tool for tightening the screw. The shank is in mechanical communication with the head and comprises threads. A coating of hard carbon is applied to the seating surface of the screw head and to at least a portion of the shank. The dental implant comprises a root portion and a threaded socket engaged with the threads of the retaining screw. The abutment is substantially irrotationally coupled with the implant and comprises a seating surface. The seating surface of the abutment is engaged with the seating surface of the head of the retaining screw. Advantageously, the hard carbon coating permits fastening of the abutment to the dental implant with an increased clamping force.
In accordance with one embodiment of the present invention, an abutment securement system to fasten dental components in a stack is provided. The abutment securement system comprises a film of diamond-like carbon (DLC) applied to a seating surface of a screw and/or to a seat of an abutment. Advantageously, this reduces friction between the seating surface and the seat, and thereby provides improved preloading of the screw.
In accordance with another embodiment of the present invention, a dental implant stack for supporting an oral restoration is provided. The dental implant stack generally comprises an implant, an abutment and a dental screw. The implant comprises a body portion adapted to be received in an alveolar cavity and an internal threaded bore. The abutment comprises a through cavity having a shoulder formed therein and is seated on the implant. The dental screw comprises a threaded portion engaged with the threaded bore of the implant and a cap having a seating surface abutting against the shoulder of the abutment. A coating of amorphous hard carbon is provided on the seating surface of the dental screw to reduce friction. Desirably, this provides a high clamping force between the implant and the abutment.
In accordance with yet another embodiment of the present invention, a dental prosthetic assembly is provided. The dental prosthetic assembly generally comprises a dental implant, a dental component and a securement bolt. The dental implant is osseointegrated in a jawbone and has a threaded socket originating from a top end. The dental component is in abutting contact with the implant and has an internal seating surface. The securement bolt has a threaded portion engaged with the threaded socket of the implant. A film comprising amorphous diamond is provided on the threaded portion of the securement bolt and/or the threaded socket of the implant. This reduces the coefficient of friction between the threaded portion of the securement bolt and the threaded socket of the implant.
In accordance with a further embodiment of the present invention, a dental prosthetic implant system for supporting an artificial tooth is provided. The dental prosthetic implant system generally comprises an implant, an abutment, a washer and a screw. The implant has a post at a top end and a threaded bore originating therefrom. The abutment has an internal passage with a shoulder therein and a socket at a bottom end substantially irrotationally engaged with the post of the implant. The washer is seated on the shoulder of the abutment passage and has a conical seating surface. The screw comprises a threaded portion engaged with the threaded bore of the implant and a head having a tapered surface abutting against the conical seating surface of the washer. This resists or reduces the tendency of screw loosening.
In accordance with one embodiment of the present invention, a method of forming a dental stack for supporting a prosthesis is provided. The method comprises the step of placing a first dental component having an internal cavity on a second dental component having a threaded bore. A retaining screw is inserted through the cavity of the first dental component to threadably engage the threaded bore of the second dental component. The retaining screw has a coating of hard carbon formed thereon to reduce friction. The retaining screw is then tightened to a predetermined or preselected torque to fasten the first dental component to the second dental component.
In accordance with another embodiment of the present invention, a method of increasing the preload on a screw used to secure dental components is provided. The method comprises the step of seating a dental component on an implant. The dental component has an internal shoulder. The implant has a threaded socket and is osseointegrated in a jawbone. A screw is threaded in the threaded socket of the implant and the head of the screw is seated against the shoulder of the dental component. The head of the screw and/or the shoulder of the dental component have/has a diamond-like carbon coating to reduce friction. The screw is then torqued using a tool to a predetermined or preselected load.
In accordance with yet another embodiment of the present invention, a method of making a retaining screw for securing and assembling dental components in a stack is provided. The method comprises the step of providing a cap portion on the retaining screw so that the cap portion has a seating surface and a cavity adapted to receive a tool for tightening the screw. A shank portion is provided on the retaining screw so that the shank portion has threads thereon. An amorphous hard carbon coating is formed on at least the seating surface of the cap portion to provide improved preloading.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects and advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.